Bosch EasyControl displays error C5 and fails to control domestic hot water (DHW) or cylinder temperature.

Safety first: Turn off power to the boiler and any cylinder controls before removing covers or touching wiring.

• 1. Confirm DHW configuration: In the EasyControl installer menu, check whether DHW control is enabled and whether the system is set to combi boiler or system boiler with cylinder. Correct any misconfiguration.
• 2. Inspect DHW sensor: Locate the DHW or cylinder temperature sensor (usually clipped into a pocket on the cylinder or attached to the DHW outlet on the boiler). Check for loose connections, damaged cable, or corrosion at terminals.
• 3. Test sensor continuity: With power off, disconnect the sensor and measure resistance with a multimeter. Compare the reading to the manufacturer’s temperature/resistance chart. Replace the sensor if readings are out of range or open-circuit.
• 4. Check wiring route: Ensure the sensor cable is not run alongside mains cables that could cause interference, and that it has not been cut or crushed.

Professional help: If the sensor and wiring are sound but C5 remains, the boiler’s DHW control board or the EasyControl DHW control logic may be faulty and should be checked by a heating engineer.